THE HISTORY OF LIFE 169 



rocks and the sedimentary rocks. These latter rocks 

 appear to have been partly changed into igneous rocks 

 after they had been deposited, 1 and this may have been 

 done by great heat and pressure. 



Now it is possible, with the evidence before us, to 

 believe that the condition of igneous rocks may 

 merely arise from certain matter, which has great 

 affinity molecule for molecule, and in the combina- 

 tion great heat was evolved, and under the influence 

 of this heat crystallization took place. It matters not 

 whether the earth were once, as a whole, in an incan- 

 descent state, and subsequently slowly, very slowly, 

 cooled ; all we have to understand is, that, at the base 

 of all rocks, there are these ancient igneous rocks, 

 formed probably under great pressure and at a very 

 high temperature. 



Of the thickness of these igneous rocks we know 

 absolutely nothing. We cannot get to the bottom of 



1 "In this section" (the Metamorphic rocks) "is comprised a 

 series of rocks which present a remarkable system of divisional planes 

 that are not original but have been superinduced upon them. At the 

 one end stand rocks which are unmistakably of sedimentary origin, 

 for their original bedding can often be distinctly seen, and they also 

 contain organic remains similar to those found in ordinary unaltered 

 sedimentary strata. At the other end come coarsely crystalline 

 masses, which in many respects resemble granite, and the original 

 character of Avhich is not obvious." (" Text-Book of Geology," Sir 

 Archibald Geikie, F.R.S., 3rd edition, 1898, p. 175.) 



This evidence tends to the view that igneous rocks may be changed 

 sedimentary rocks changed by heat and great pressure. If this 

 alteration be simply caused by chemical reaction, it is not necessary 

 to suppose the earth was originally in an incandescent white or red- 

 hot condition. If this view is true, then the age of the world 

 during which life existed must be very vastly increased over that 

 which has been estimated. 



